1. Field of the Invention
This invention relates to a wet friction material that generates a torque by applying a high pressure to an opposite surface while being immersed in an oil. In detail, this invention relates to a segment-type friction material made by joining friction material substrates that were cut into a segment piece onto one or both of opposite surfaces of a core metal of a flat ring shape along an entire circumference thereof with an adhesive. Otherwise, this invention relates to a press-type friction material made by pressing one or both of opposite surfaces of a friction material that has friction material substrates joined onto one or both of opposite surfaces of a core metal of a flat ring shape along an entire circumference thereof with an adhesive so as to form a plurality of oil grooves in a radial direction.
2. Description of the Related Art
In recent years, a segment-type friction material 21 as shown in FIG. 11 is under development as a wet friction material in order to improve material yield with resultant cost reduction and to lessen drag torque with resultant high fuel efficiency. FIG. 11 illustrates a plan view of a partial structure of a conventional segment-type friction material. As shown in FIG. 11, the segment-type friction material 21 has a core metal 2 of a flat ring shape and friction material substrates 23. The friction material substrates 23 are cut into a segment piece corresponding to and arranged along the flat ring shape. A set of the segment-piece-shaped friction material substrates 23 are arranged and joined with an adhesive on a front surface of the core metal 2 side by side along an entire circumference of the core metal 2 with a gap to be an oil groove 26. Another set of the segment-piece-shaped substrates 23 are joined with the adhesive to a rear surface of the core metal 2, too. Such segment-type friction material 21 is applicable to a friction material clutch device that has a single or plural friction plates and that is used in an automatic transmission, which may be referred to as “AT” hereafter, of automobiles or motorcycles or the like.
As an example, a wet hydraulic clutch is used for an automatic transmission of an automobile or the like. The wet hydraulic clutch has a plurality of segment-type friction materials and a plurality of separator plates laid alternately on each other. Then, both the plates are contacted with a hydraulic pressure to transmit a torque. A lubricating oil or an automatic transmission fluid, which may be referred to as “ATF” hereafter, is supplied to between both the plates for the purpose of absorbing a frictional heat generated when they are shifted from a disengaged state to an engaged state or preventing abrasion or the like. However, a distance between the segment-type friction material and the separator plate is set small in order to increase response of the hydraulic clutch. Moreover, a total area of an oil passage provided on the segment-type friction material is restricted in order to assure a sufficient torque transmission capacity at the time of clutching operation of the hydraulic clutch. Consequently, it is hard to discharge the lubricating oil remaining between the segment-type friction material and the separator plate at the time of clutching operation of the hydraulic clutch. As a result, there was a problem that a drag torque was generated by the hydraulic oil caused by a relative rotation of both the plates.
In view of the above, according to an invention described in Japanese Laid Open Patent Publication No. 2002-340071, a wet friction plate as a segment-type friction material has two-tiered segment pieces arranged at an inner circumference and an outer circumference thereof. At least part of an inner peripheral edge of the segment piece is inclined outwardly in a radial direction toward a radial inner end of a radially extending oil passage that faces a lagging side in a rotating direction. Thus, in the technique of 2002-340071, it is intended to lead the lubricating oil effectively to the radial inner end of the radially extending oil passage.
However, in the technique described in the above patent publication No. 2002-340071, the radially extending oil passage is narrow and extends with a same width. In contrast, a test was conducted to show a relation between a groove width ratio (a/b) of an outer opening “a” and an inner opening “b” of the radially extending oil passage and a drag torque reduction rate in the conventional wet friction material 21 shown in FIG. 11. Then, it was confirmed that the wet friction material only had a torque reduction rate of about 30% in case the groove width ratio (a/b) equaled 1.0 or the oil passage extended with a same width. That is, the wet friction material only had a half effect of the case where the groove width ratio (a/b) equaled 3.0 or 4.0. Moreover, in the technique described in the patent publication No. 2002-340071, the segment pieces cut into small pieces are joined with an adhesive in two tiers at the inner circumference and the outer circumference. Therefore, there is a problem that the drag torque is not reduced enough by the lubricating oil, though it involves effort to produce and increases costs.